The present invention relates to improvements in or relating to marine pipelaying methods and apparatus. The invention is particularly concerned with controlling the ovality of rigid pipeline during pipelaying operations in which the pipeline is plastically deformed during bending of the pipeline around an arcuate path and is subsequently straightened prior to laying.
The invention will be discussed herein with particular reference to rigid steel pipe, but is also applicable to rigid pipe formed from other materials.
Rigid steel pipe is manufactured to a nominal circular diameter. However, in practice the pipe will not be perfectly circular along its entire length, but will exhibit variations in ovality, within defined tolerances. Subsequent processing of the pipe, such as by bending, will cause further variations in ovality. In the context of marine pipelaying, ovality affects the ability of the pipe to resist hydrostatic pressure, particularly at extreme water depths, and it is important that the ovality of the pipe as finally laid does not exceed predetermined limits. Ovalisation of the pipe may become particularly significant where the pipe is being laid in relatively great water depths requiring unusually high tension to be applied to the pipeline, thereby increasing the forces exerted between the pipeline and an underlying pipe bearing surface, prior to the launch of the pipeline from the vessel.
Ovality may be defined as: EQU Ovality=Dmax-Dmean/Dmean;
where Dmax is the maximum diameter of the pipe and Dmean is the mean diameter of the pipe. In a given length of pipe, the angle formed between the maximum diameter (or "major axis") and a reference plane extending through the longitudinal axis of the pipe may vary along the length of the pipe. Typically, the maximum diameter may rotate along the length of the pipe so that the ovality spirals along the pipe.
In the present discussion the following conventions will be employed:
where the major axis of the pipe lies along the reference plane the ovality will be referred to as a positive ovality; in this case, the diameter along the reference plane is greater than the nominal circular diameter; PA1 where the major axis lies at right angles to the reference plane the ovality will be referred to as a negative ovality; in this case the diameter along the reference plane will be less than the nominal circular diameter; PA1 in cases where the pipe is being bent around an arcuate path the reference plane will be the plane of curvature of the pipe.
It can readily be seen that where the pipe exhibits positive ovality prior to bending of the pipe, the ovality of the pipe will be reduced by such bending, since the process of bending will tend to increase the diameter at right angles to the plane of bending and to reduce the diameter in the plane of bending. Conversely, where the pipe exhibits negative ovality prior to bending, the ovality will be increased by bending.
Where the pipe is bent elastically, it can be expected to return to its original ovality when the bending forces are removed. However, where the pipe is plastically deformed during bending and is subsequently straightened, the pipe will not fully recover its original ovality and there will be a net residual change in its final ovality as compared with its ovality prior to bending. Where the original ovality is positive, the net residual change will result in a reduced positive ovality. Where the original ovality is negative, the net residual change will result in an increased negative ovality. In the latter case it can be seen that there may be cases where a length of pipe which is within predetermined ovality tolerances prior to bending might exceed such tolerances after bending and straightening owing to the net increase in negative ovality. In the former case the net decrease in positive ovality will generally be desirable.
It will be understood that, where the pipe is bent against a supporting surface, there will also be a degree of flattening of the pipe. Herein, such flattening is considered to be a component of the overall ovalisation.
The present invention is primarily concerned with controlling pipeline ovalisation in marine pipelaying operations where the pipe is subject to plastic deformation during bending and subsequent straightening in the course of the laying operation. Such plastic deformation occurs both in pipelay systems where a continuous length of pipeline is assembled onshore and is spooled onto a reel, the pipe being unspooled from the reel, plastically bent around an arcuate path to a desired launch angle and straightened as it is laid from the lay vessel. Plastic deformation also occurs in a variation of "stovepipe" operations in which joints of pipe are assembled into a continuous pipe on board the vessel and in which the assembled pipe is plastically bent around an arcuate path and subsequently straightened in order to achieve a desired launch angle of the pipe from the vessel. Reel pipelaying systems of the former type are utilised by the vessel "Stena Apache" and are described in detail in, for example, U.S. Pat. Nos. RE30846, 4,260,287, 4,230,421 and 4,297,054. Pipelay systems of the latter type are described in co-pending International Patent Applications Nos. PCT/GB95/00573 and PCT/GB95/00574 in the name of the present assignee.
In both of these cases, the arcuate path around which the pipe is bent is typically defined by a plurality of pipe support pads. In order to prevent relative movement between the pipeline and the pipeline contacting portions of the pads, such pads might be mounted on endless-belt type tracks or on a rotatable wheel-like structure, such that the pads move with the pipe, or might be static and include pipe-contacting roller bearings. Arrangements of these general types are known in the art. In the case of the rotatable wheel-like structure referred to above, the pipe supporting surface might comprise a continuous, circular rim of the structure, rather than a plurality of discrete pads. References to "support pads" and "support elements" used herein will be understood to include such arrangements.
When a pipeline contacts such support pads under tension, the reaction force between the pipeline and the support tends to deform the pipeline towards negative ovality, and may also result in the formation of flats on the pipeline surface. It is an object of the present invention to provide improved pipeline support pads which reduce the tendency for ovalisation of the pipeline and/or reduce flat-formation.